mirror mesh on opposite side of x axis
There's an orange dot with a black outline that is more visible when you are in object mode. That's your object's origin. The mirror modifier reflects over that origin point. If you'd like to change the origin point, you can do so a few ways. The way I'd do it is to select the top-most vertex alone. Shift+S and select Cursor to selected. Then go back into object mode and hit Ctrl + Alt + Shift + C and select Origin to 3D Cursor.